If you're familiar with the tent-pole programming analogy, you'll understand that some shows in the library are considered important to watch. Pardon the pun, it's clear that Attack on Titan definitely stakes its claim.
Attack on Titan began as a one-shot manga in 2006 when its creator, Hajime Isayama, was just 20 years old, and has been a huge success ever since. The manga will join the serialization lineup in 2009 as part of the monthly Bessatsu Shōnen Magazine, and is close to the top 10 best-selling manga of all time.
An anime adaptation will follow in 2013 that will span three seasons over several years, with its fourth and final season set to premiere this December. Of course, you'll be able to watch all four seasons here at AnimeLab, as well as the live action adaptation and comedy branch... More on that in a bit.
Set in a fictional land resembling Europe in the early 1900s, Attack on Titan tells the story of humanity's last enclave. Giant, mindless beasts roamed the plains, causing tremendous destruction in their wake. These are the Titans. Little is known about their true origins or motives, apart from their insatiable hunger for human flesh.
In a desperate attempt to save some semblance of peace, walls were erected and cities built within them. To reduce extinction even further, interior walls were installed, and in turn, a hierarchy was created. The outermost wall, Wall Maria, is where the majority of the population lives. These are your commoners and blue-collar workers, and even among them, smaller communities have been strategically placed on the margins. Wall Rose came next, acting as protection and refuge, while Wall Sina housed the royal family and the remaining members of society.
Because of this immense effort, the dwindling members of humanity enjoy a century of frail tranquility, unafraid of the predators living nearby while ultimately living as prisoners of their own creation.
This is until a large and aggressive unit of Titans irregularly breaks a hole through Wall Maria, and in doing so, wipes out most of the population. Among those affected are Eren Jaeger and his friends, and he vows to take revenge on the beast that stole everything from him.
Refrence by: https://www.animelab.com/blog/the-beginners-guide-to-attack-on-titan/